In the related art, as one of systems which purify exhaust gas of an automobile, there has been known a selective catalytic reduction (SCR) system that reduces NOx to nitrogen and water using ammonia, and zeolite having a copper-carried CHA (chabazite) structure is paid attention as zeolite achieving SCR catalysis.
In this SCR system, a honeycomb unit, in which a large number of through holes allowing the exhaust gas to pass therethrough are arranged in parallel to extend in a longitudinal direction, is used as an SCR catalyst carrier. For example, as one that aims to enhance heat resistance and endurance in a case of being used as an SCR catalyst carrier, zeolite having a CHA structure is disclosed in Patent Literature 1 in which a composition ratio of SiO2/Al2O3 is less than 15 and an average particle size is 1.0 to 8.0 μm.
On the other hand, zeolite having a CHA structure is disclosed in Patent Literature 2 in which a composition ratio of SiO2/Al2O3 is less than 15 and the content of alkali is less than 3% by mass. In this case, however, the surface of a honeycomb unit substrate made of another substance needs to be coated with the CHA-structured zeolite to obtain an effect as an SCR catalyst.
A technique is disclosed in Patent Literature 3 in which zeolite is extrusion-molded together with an inorganic binder and the like into a honeycomb shape in producing of a honeycomb unit to be used as an SCR catalyst.
A microporous crystalline material is disclosed in Patent Literature 4 which contains a metal containing chabazite having a crystal size greater than 0.5 microns and a silica-to-alumina ratio (SAR) greater than 15, wherein the metal containing chabazite retains at least 80% of its initial surface area and pore volume after exposure to a temperature equal to or lower than 900° C. in the presence of water vapor of 10 vol % or more for up to 1 hour.